Note 68 — Chapter IV · (continued 2)
Reading edition
The main crux of this passage is "fit quantity of syllables." Quantity in such a context suggests syllabic length; and one recalls the sonnet to Lawes—
not to scan With Midas' ears, committing short and long.
But, on the other hand, Mr. Robert Bridges has made it almost if not quite certain that Milton counted syllables, and therefore the phrase would mean "ten syllables to a line," proper allowance being made for elision. Since both interpretations agree pretty well with Milton's practice, one cannot be sure which he had in mind.